The present invention relates to a cardiac assist device and more particularly to an implantable artificial left ventricle including a continuously operating pump and hydraulic valve system which assists the heart by intermittently pumping blood in synchronization with operation of the heart.
Congestive heart failure represents an enormous national public health concern, with a prevalence of 4.9 million cases and an incidence of more than 400,000 cases a year. Some 50,000 people a year die of heart failure and it is a contributing factor in 250,000 additional deaths a year.
Congestive heart failure accounts for more than hospitalizations a year, at a hospital cost of $18.8 billion. The direct and indirect cost of treating this disease has been estimated at $64 billion a year.
Congestive heart failure is the only form of heart disease that is increasing in the United States. It will undoubtedly continue to do so, as more and more victims of coronary occlusions survive and their longevity is increased. However, with each such event, the heart muscle is further injured and the resulting scar tissue decreases the ability of the left ventricle to perfuse the body. Thus, higher survival rates and increased longevity lead to an increased number of people surviving long enough for congestive heart failure to become a likelihood. The development of a device to augment or replace the pumping ability of the left ventricle would prevent congestive heart failure.
During the last twenty years, a variety of mechanical devices have been developed to support circulation. These devices, known as ventricular assist devices, usually aimed at the left ventricle, (sometimes known as LVAD's) have been used primarily for temporary short-term circulatory support as a bridge to cardiac transplantation. The success of these devices has led to the understanding that LVAD's may well be developed as a long-term therapy for patients with end-stage heart disease. Further, the utilization of LVAD's for patients with chronic heart disease has been recognized as a permanent treatment option for many patients.
The existing implantable cardiac assist ventricles either modify the anatomy of the heart (apico-aortic assist), replace the heart entirely, or use an atrial to aortic pump which provides a constant blood flow.
The present invention takes a different approach. It is essentially an implantable artificial ventricle that leaves the heart intact, improves the function and nutrition of the cardiac muscle, and normalizes the perfusion of the entire vascular system. Our device is designed such that in the event of mechanical failure, it will not impede pre-implantation operation of the heart. Hence, the patient is not medically worse off than he would have been without the device, if it were to fail.
The present invention utilizes a counterpulsation pumping action and a unique valve structure. The device pumps blood during each cardiac diastole but not during cardiac systole. The result is that the work of the heart is decreased during systole. Coronary blood flow and perfusion of the heart itself are increased during cardiac diastole, when the transmural cardiac resistance is at its nadir. The increased coronary flow during cardiac diastole has an angiogenic effect, i.e., it promotes the formation of new coronary collateral channels. This beneficial effect has been shown to be associated with an improvement in left ventricular function, and a significant decrease in angina.
Our invention provides for long term implantation of a counterpulsating chamber in the descending aorta, either above the diaphragm or below the inferior mesenteric artery, which will assist the failing or failed left ventricle. Candidates for such an assist system would, at first, be NYHA Grade IV patients, in chronic failure and bedridden. As the procedure gains acceptance, it would be implanted in somewhat less severely ill patients.
We utilize a closed hydraulic fluid system which includes a hydraulic pump. The pump has a continuously operating voice coil linear electric motor which drives a hydraulic piston. The motor and piston are preferably constructed as a single sealed unit, with the motor submersed in hydraulic oil, so as to be supported on its own hydrodynamic oil bearing. Such a pump is disclosed in detail in U.S. Pat. No. 5,360,445, entitled “Blood Pump Actuator,” issued to Goldowsky on Nov. 1, 1994.
As indicated in the Goldowsky patent, this arrangement greatly increases the life of the pump by eliminating wear at the bearings. The hydraulic fluid is selected to have a very low viscosity and very good lubrication capability.
Hydraulic valves are utilized to convert the continuous fluid flow output of the pump into the intermittent fluid flow needed to provide the desired counterpulsed blood flow. The valves are controlled in accordance with electrical signals from the heart such that blood is pumped only during cardiac diastole.
The valves are situated in a multi-valve body or chamber. After closure of the aortic valve, hydraulic fluid is pumped into a variable volume chamber which expands, causing blood to be forced into the arteries. During the next cardiac systole, fluid is pumped from the chamber, into a fluid reservoir which accumulates the fluid during pump diastole, such that no force is exerted by the device on the circulatory system.
A bypass is employed to relieve the fluid build-up in the chamber, if the pressure rises above a given level. This might take place, for instance, if a pause occurs in the operation of the valves due to a pause in the rhythm of the heart. The bypass permits the pump to continue to operate without overloading the closed hydraulic system.